1. Field of the Invention
This invention relates to a seed-crystal holder installed in single-crystal pulling devices, which is used for holding seed crystals while applying a magnetic field to a melted liquid, such as silicon melted liquid, and pulling single crystals therefrom.
2. Description of the Related Art
Conventionally, in the process of pulling single-crystal ingots such as silicon single-crystal ingots, a seed crystal 10 is brought into contact with melted liquid 6 and magnetic field is applied laterally to melted liquid 6 by means of the magnets 8, then the seed crystal 10 is rotated to pull the semiconductor ingot 60.
The essential requirements for a seed-crystal holder adapted to hold seed crystals are superior heat endurance and adequate weight. If it has not enough endurance, single crystal contaminated with the material of the seed crystal holder. If it is too light, it is not capable of obtaining a stable rotation of the seed crystal holder. Several materials such as molybdenum and graphite meet the above requirements. However, the drawback of such conductive materials as molybdenum and graphite is that a braking torque having a tendency to stop the rotation of the seed-crystal holder is produced along with the electric current induced by electromagnetic induction of magnetic field. The magnitude of the electric current induced depends on the magnetic-flux density of the magnet field, the rotation speed of the seed crystal, and the conductivity of the material of seed crystal holder. The higher magnetic-flux density, rotation speed and conductivity, the larger magnitude of the lectric current and the breaking torque.
There are several ways to overcome the brake torque problem, such as "SEED-CRYSTAL HOLDER FOR SINGLE-CRYSTAL PULLING DEVICES WITH MAGNETIC FIELD APPLIED THEREON" disclosed in EXAMINED JAPANESE PATENT PUBLICATION HEI 5-11077. The body of the seed-crystal holder is made of non-conductive materials such as ceramic, and thus electromagnetic induction is eliminated.
However, if the body of the seed-crystal holder is made of non-conductive materials such as ceramic whose specific gravity is less than that of molybdenum, then the weight of the seed-crystal holder becomes about 1/3 of that of a conventional one. Such light-weight seed-crystal holders is not applicable to handle the increased weight load resulting from the enlargement of the diameter of each semiconductor ingot, and steady rotation can not be obtained.
In other words, the cable for suspending the seed-crystal holder must be thicker accompanying with increased diameters of semiconductor ingots. If a light seed-crystal holder is installed on a cable that is thicker than conventional one, the cable can not be kept in the state of being vertically suspended. That is the cause of rotational wobbing of the seed crystal holder. Then, the ingot grown becomes bent, and it proves impossible to control of its diameter.